Crystalline structure and physical properties of ship superstructure spray ice

Abstract

In February and March 1990 the US Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL) made measurements of superstructure ice on a US Coast Guard cutter in the Bering Sea. Twenty–three ice samples were removed from bulkheads, decks and icicles during two icing events. Ice crystal measurements included crystal size, shape, orientation, brine–pocket location, size and shape, internal layering, and air–bubble sizes. Ice property measurements included salinity, density and temperature, with computed estimates of air and brine volume. This paper describes crystal and physical properties of the accreted ice and their relationship to ice sample position on the ship. Texturally, accreted ice resembled frazil ice that forms from the consolidation of freely nucleated ice crystals in sea water. This resemblance is also reflected in bulk salinities, ranging from 24‰ to 7‰, compared with frazil formed during the initial stages of freezing of sea water, where bulk salinities can exceed 10‰. Crystalline structures of accreted ice ranged from rounded to polygonal. Generally, rounded crystals would be expected for ice formed from sea–spray droplets, polygonal crystals may be attributed to thermally driven modification. No trend towards reorientation of crystallographic c–axes in either freshly accreted or thermally modified ice was observed. Mean crystal sizes ranged from 0.56 mm to 1.15 mm, with even larger crystals in icicles. Ice salinity averaged ca .12‰ on bulkheads and ca .21‰ on decks. Ice densities ranged from 0.69 to 0.92 Mg m −3 and were generally higher on decks. Bulkhead ice had larger computed total porosity and air volume and lower brine volume than deck ice. Samples taken from decks and bulkheads generally compared well with Russian and Japanese measurements.